1. Field of the Invention
The present invention relates to a comparator circuit.
2. Description of Related Art
Generally, a vehicle-installed rotation detector used as a crank angle sensor, a cam angle sensor, a wheel speed sensor and the likes is configured to detect rotation of a detecting gear on the basis of changes of the direction of a magnetic flux flowing between the detecting gear and a bias magnet by use of magneto resistive elements utilizing the fact that the direction of the magnetic flux when the bias magnet faces one of the tooth roots of the detecting gear is different from that when the bias magnet faces one of the tooth peaks of the detecting gear.
Such a rotation detector has a comparator circuit as shown in FIG. 3. In this drawing, an input voltage signal supplied from the side of magneto resistive elements (not shown) is amplified by an operational amplifier OP, and then applied to a signal input terminal Cs of a comparator CP. On the other hand, a reference voltage input terminal Cr of the comparator CP is applied with a reference voltage produced by dividing down a power supply voltage Vc by a first voltage dividing resistor R1 and a second voltage dividing resistor R2. The comparator CP outputs a high-level signal when the voltage of the amplified input voltage signal is lower than the reference voltage. Such a comparator circuit is disclosed in Japanese Patent Application Laid-open No. 10-293044, for example.
However, the above described comparator circuit has a problem in that it tends to erroneously output a pulse-like signal when it is used in a vehicle, because large noise emitted from a high-tension code supplying a high-tension voltage to a spark plug easily enters a power line connected to a vehicle battery.
Furthermore, the above described comparator circuit can erroneously output a pulse-like signal also when several of turn-on timings of large-power consuming units such as headlights, an air conditioner, and a power steering unit overlap, and thereby the voltage of the power line momentarily drops greatly because of control delay of a vehicle generator.
Explanation as to how the comparator circuit outputs a false pulse is made below with reference to FIG. 4A.
Assume a case where the power supply voltage (the voltage of the power line) falls from 5V to 1V at time t1, and subsequently rises from 1V at time t2 as shown in (A) in FIG. 4A. In this case, as shown in (B) in FIG. 4A, each of the reference voltage represented by a broken curve and the voltage of the amplified input voltage signal outputted from the operational amplifier OP represented by a solid curve starts to fall at time t1, and starts to rise at time t2.
However, since the operational amplifier OP includes capacitors therein and accordingly has an integral characteristic, the falling and rising slopes of the voltage of the amplified input voltage signal become gentler than those of the reference voltage. As a result, the voltage of the amplified input voltage signal becomes lower than that of the reference voltage for a certain time period starting from time t2. This causes the comparator CP to output a false pulse at time t3 which is behind time t2 by a certain delay time (19 μs, for example) as shown in (C) in FIG. 4A.
The comparator circuit can output such a false pulse also when the power is turned on as explained below with reference to FIG. 4B. When the power is turned on, and the power supply voltage rises from 0V at time t4 as shown in (A) in FIG. 4B, each of the reference voltage represented by a broken curve and the voltage of the amplified input voltage signal outputted from the operational amplifier OP represented by a solid curve starts to rise from 0V at time t4 as shown in (B) in FIG. 4B.
However, since the operational amplifier OP has integral characteristic as described above, the falling and rising slopes of the voltage of the amplified input voltage signal become gentler than those of the reference voltage. As a result, the voltage of the amplified input voltage signal becomes lower than the reference voltage for a certain time period starting from time t5. This causes the comparator CP to output a false pulse at time t6 which is behind time t5 by a certain delay time (19 μs, for example) as shown in (C) in FIG. 4B. The comparator circuit can output a false pulse also when the power is turned off. Such a false pulse causes reduction of accuracy in detecting the crank angle, cam angle, or wheel rotational speed, for example.